Conventionally, in an automotive air conditioner, the driving force of an automotive engine has been used to drive a compressor for compressing a refrigerant. In contrast, with the recent practical use of electric vehicles and the like, what is called an electric compressor, in which an electric motor is used as a driving source for the compressor, has been developed.
In such an electric compressor, the driving torque produced by the motor is lower than that produced by the engine. Especially at the time of actuation, the driving torque required by the motor is high, so that the compressor cannot sometimes be actuated depending on the state of refrigerant at the front and rear of the compressor. After the operation has been stopped, the temperature of refrigerant decreases. If the refrigerant that has been in a gasified state at the time of operation is liquefied on the outlet side of compressor due to the decrease in temperature, the density of refrigerant is far higher than that of refrigerant in the gasified state. The reason for this is that the motor load that is going to actuate the compressor becomes excessive, so that, in a motor drive control circuit, an overcurrent protecting function for protecting the motor is triggered.
Such a phenomenon is remarkable especially when heating operation is going to be performed in a state of low open-air temperature. Accordingly, a technique has been proposed in which a dc current is supplied to the coil of motor stator to generate heat in the coil, by which the compressor is preheated (for example, refer to Patent Document 1).    Patent Document 1: Japanese Patent Laid-Open No. 2005-326054